Given the comment about Lasers earlier this
link might be of interest. Hower I think a microwave oven is about as closely related to a real Maser as a LED is related to a laser. Close but not coherent, kind of like a lot of my posts! I suppose if the oven cavity is designed for resonance then it is bordering on some type of Maser. Of interest perhaps is the way the door seals. Around the rim is a cavity that sets up a destructive supposition of the waves canceling them out so we don't get a leak. not to mention the Faraday screen in the veiwing glass. Some cooking vessels are designed to heat directly in a microwave. The browning dish for example.
Caution!
I used to put my pies in there foil dish as this would heat up directly and help warm the centre of the pie
BUT the dish must be shallow and continuous, Never put a metallic ring (gold leafed plate in etc) Especially if it already has a small gap!